15 August 2008

The fall and rise of New Zealand

There are two things that we know for certain about the geological history of New Zealand. First, the land area that is now modern New Zealand was part of Gondwanaland until about 83 million years ago. Second, modern New Zealand is isolated from other large landmasses by thousands of kilometres of ocean. What happened in between is not so clear. A recent study by researchers from Lincoln University, Geological Nuclear Sciences, Massey University and University of Otago has thrown some light on a key episode in the history of the region. The study, published in the Geological Magazine (The Waipounamu erosion surface: questioning the antiquity of the New Zealand land surface and terrestrial fauna and flora. Landis, Campbell, Begg, Mildenhall, Paterson & Trewick), has important implications for the history of New Zealand’s flora and fauna. It has long been known that after the separation from Gondwanaland, the crust under the New Zealand region (or Zealandia) started to thin, which lead to a gradual submergence of Zealandia. Most of Zealandia (an area the size of India) is now two kilometres under water. The sinking became very acute around 25 million years ago (in the period known as the late Oligocene) when maximum submergence was reached. Following this 'Oligocene Drowning', tectonic activity along the Pacific and Australian plates led to crustal thickening and uplift in the New Zealand region. There has been much debate about how much of modern New Zealand was left above water during that time.


Landis and his colleagues looked at the evidence for dry land during the Oligocene period in order to better estimate the location and proportion available for the flora and fauna to have survived on. One major piece of evidence that they used was the Waipounamu erosion surface. These surfaces are flat areas in the landscape that represent wave cut platforms created as Zealandia was slowly inundated by sea. They also used sedimentary deposits, terrestrial fossil sequences and geomorphological evidence to reach a conclusion that there was no hard evidence for ANY land during the late Oligocene. The authors argue that the New Zealand region may have completely submerged or, at most, persisted in tiny remnants of land during the peak inundation. This has obvious implications for New Zealand’s biology as it suggests that all, or at least the vast majority, of species have colonised New Zealand over the last 22 million years across an ocean gap rather than simply persisting here for 80 million years in splendid isolation.

So much for the geology, what does the biology tell us? We know that the New Zealand biota can change very quickly. New Zealand’s forest composition has changed remarkably over the last tens of thousands of years. Fossils from a lake bed at St Bathans from 16 million years ago show evidence of a more tropical fauna complete with crocodiles, parrot and bat species not found in New Zealand today and even a mammal bone! So 22 million years is long enough for all sorts of things to happen. Molecular data, which can estimate when common ancestors lived, also overwhelmingly shows that closest relatives to taxa living in New Zealand (generally found in Australia) diverged from them well within the last 20 million years, implying that they arrived and colonised here. The only biological evidence that challenges the idea of submergence is that of ancient taxa, like tuatara, leiopelmatid frogs and the recently extinct moa. Their presence in New Zealand and nowhere else suggests that they inhabited Gondwanaland and were isolated here for 80 million years. Tuatara and the frogs are both groups that can persist on small islands and may represent true ‘ghosts of Gondwana’ that made it through the submergence. Moa are species of ratites and we know that another ratite group, the kiwi, did successfully colonise New Zealand. Perhaps moa did the same.


jon sullivan said...

This is fascinating stuff. It is certainly a strong challenge to long-held views about New Zealand's ancient and isolated Gonwanan species.

You conclude that there was either no land or only "tiny remnants of land". To what extent can we rule out the presence of substantial areas of land elsewhere in and around Zealandia? Places submerged today may have once had their day in the sun. Has there been adequate offshore drilling to rule out such places, or put an upper size limit on such places? (Perhaps I should read the paper carefully to find out!)

Flightless moa, even small ones, aren't going to have colonised New Zealand across thousands of kilometers of open ocean. That's just crazy. There simply must have been land hanging out somewhere closer.

Adrian Paterson said...

Hopefully the Marsden Fund will be kind to our current application as we want to do a similar study in New Caledonia. The existing geological evidence suggests that New Caledonia was drowned between 40-30 million years ago (NZ may have been a refuge for the Zealandian fauna and flora) but was above water when NZ was inundated (so NC might have been a refuge for the Zealandian biota during this period). There is also some evidence for islands between the areas during this time. So you are right, species could have colonised from much closer areas than we see today.